Many portable digital devices incorporate at least one display screen to exchange information with users. Sample displays may incorporate liquid crystal display (LCD) technology, which is one of the most widely used display technologies. An LCD generally includes a backlight module, such as a light emitting diode (LED), a bottom polarizer, a TFT glass, liquid crystal molecules, a color filter glass, and a front polarizer. Light emitted from the backlight module passes through all the LCD components to generate desired images. The display may also include organic light emitting diode (OLED), plasma, and the like.
Displays may vary from panel to panel, even in the same product line or manufacturing line, due to inherent properties of the displays. For example, variations exist in LCD components, such as backlight variations due to LED wavelength and phosphor concentration, color filter thicknesses, and the like. Thus, each display may have slightly different color characteristics, white points and the like.
A white point of a display panel is defined by a pair of chromaticity values (x and y) that represent the color produced by the panel when the panel generates all colors at full power. For example, when red, green, and blue channels for a display panel are all active at full power, the chromaticity values, as measured in Cartesian coordinates x and y with respect to a chromaticity diagram, are the white point of the display panel. FIG. 1 illustrates sample white point variations among display panels of the same type, where a horizontal axis and a vertical axis correspond to chromaticity coordinates x and y, respectively. Each display panel has a native or original white point, which is measured without any correction. These representative values, are meant as examples and not limiting, other display may generate different data points.
The white point discrepancies shown in FIG. 1 may be corrected based upon a display white point calibration. White point calibration uses a target white point, which may be the white point corresponding to the D65 illuminant of the International Commission on Illumination (CIE). Any suitable target white point may be used. For example, each panel is tuned to have the same target white point by adjusting display control settings such as gain values for red, green, and blue channels individually during the white point calibration. After the white point calibration, the deviation among different panels may be reduced such that the white points of the panels would be within a range of target chromaticity coordinates (x0, y0). If a panel, after the white point calibration, still has a white point beyond the specified range from the target chromaticity coordinates, the panel is considered to fail the calibration and may be sent to failure analysis or further diagnostics.
However, users may still observe color differences between different panels although the white points of the different panels are calibrated to have the same target. Therefore, it may be useful to develop methods for further improving color uniformity.